The present invention relates to a controller for an internal combustion engine, and more particularly, to a controller for controlling in a variable manner the valve timing of engine valves that are opened and closed in accordance with the rotation of a camshaft.
To increase output or to improve emissions, an internal combustion engine for a vehicle or the like may be provided with a valve timing controller for optimally varying the valve timing of the internal combustion engine. Such a valve timing controller includes a rotor that rotates synchronously with a crankshaft of the internal combustion engine. The rotor accommodates a movable member, which is connected to a camshaft of the internal combustion engine, so that relative rotation of the movable member is enabled. An advance pressure chamber and a delay pressure chamber are defined on opposite sides of the movable member in the rotor. The advance and delay pressure chambers are connected to an oil pump through an oil control valve (OCV). The valve timing controller adjusts the duty ratio of the voltage supplied to the oil control valve to control the pressure difference between the pressure chambers.
The pressure difference between the pressure chambers changes in accordance with the voltage duty ratio. The movable member rotates relative to the rotor in accordance with the pressure difference between the pressure chambers. As a result, the rotational phase of the camshaft relative to the rotor changes to the advance side or the delay side. In this manner, the valve timing controller changes the rotational phase of the camshaft to vary the valve timing of the internal combustion engine.
Such a valve timing controller includes a lock mechanism for locking the camshaft at a rotational phase corresponding to a predetermined timing. This fixes the valve timing at a predetermined timing when the hydraulic pressure is low, such as when the engine is started. The lock mechanism includes, for example, a lock hole in the rotor and a lock pin arranged in the movable member. The lock mechanism further includes a spring for urging the lock pin toward the lock hole. The valve timing controller provided with such a lock mechanism locks the valve timing at a predetermined timing by an engagement of the lock pin and the lock hole. The valve timing controller separates the lock pin from the lock hole when changing the valve timing.
In such a lock mechanism, the abutment force acting between the peripheral surface of the lock pin and the wall of the lock hole decreases as the relative rotation force of the movable member relative to the rotor decreases. In this case, when the lock pin moves away from the lock pin, the friction force or resistance force that resists separation of the lock pin from the lock hole is small. The hydraulic pressure state in which the resistance force is smallest relative to the force acting to separate the lock pin from the lock hole is referred to a “hydraulic pressure state in which unlocking occurs most easily.”
To reach the “hydraulic pressure state in which unlocking occurs most easily,” the duty ratio of the voltage supplied to the OCV is set so that the valve timing is smoothly and quickly unlocked. However, the appropriate actual duty ratio for reaching the optimal hydraulic pressure state (hereinafter referred to as optimal duty ratio) is affected and is fluctuated by the intake pressure of the oil pump and the viscosity of the operating oil. Accordingly, when the optimal duty ratio is set as a fixed value, it is difficult to obtain the optimal hydraulic pressure state.
It may be considered that a holding duty ratio for holding the rotational phase of the camshaft is suitable for obtaining the optimal duty ratio. Accordingly, Japanese Laid-Open Patent Publication No. 2002-161765 describes grading the duty ratio of the voltage supplied to the OCV from the holding duty ratio so that the camshaft relatively rotates toward the advance side. Japanese Laid-Open Patent Publication No. 2002-161765 describes grading the voltage duty ratio from a limit value for relative rotation of the camshaft toward the delay side (lower limit value of variable range) via the holding duty ratio to a value for relative rotation the camshaft toward the advance side. In comparison to when the optimal duty ratio is a fixed value, this increases the possibility the voltage of the duty ratio supplied to the OCV matches the optimal duty ratio.
However, in Japanese Laid-Open Patent Publication No. 2002-161765, the duty ratio is graded from the holding duty ratio so that the camshaft relatively rotates toward the advance side. Thus, when the optimal duty ratio is a duty ratio that relatively rotates the camshaft toward the delay side, the duty ratio of the OCV does not match the optimal duty ratio. Further, in Japanese Laid-Open Patent Publication No. 2002-161765, the value at which grading starts may be set to the lower limit value of the variable range for the duty ratio of the OCV (one of the two limit values of the variable range). Accordingly, when the optimal duty ratio is relatively far from the lower limit value of the variable range (e.g., when the optimal duty ratio is close to the holding duty ratio), it takes a long time for the duty ratio to reach the optimal duty ratio from when grading is started. The prolonged grading delays unlocking, that is, delays the timing at which the varying of the valve timing starts.